Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Talk360 - Talk360 is a best-in-class communication utility that offers a convenient way to connect with loved ones globally.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FluffyChat - Open. Nonprofit. Cute ?
Sipdroid - Free and open-source SIP/VoIP Client
Textnow.com - It lets you text your friends, aka my only source of happiness